Sum of Left Leaves

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404.Sum of Left Leaves

Given the root of a binary tree, return the sum of all left leaves.

A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

Example 1:

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Input: root = [3,9,20,null,null,15,7]
Output: 24
Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.

Example 2:

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Input: root = [1]
Output: 0

深度优先遍历：

时间复杂度：O(n)

空间复杂度：O(n)

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class Solution {
public:
    bool isLeafNode(TreeNode* node) {
        return !node->left && !node->right;
    }

    int dfs(TreeNode* node) {
        int ans = 0;
        if (node->left) {
            ans += isLeafNode(node->left) ? node->left->val : dfs(node->left);
        }
        if (node->right && !isLeafNode(node->right)) {
            ans += dfs(node->right);
        }
        return ans;
    }

    int sumOfLeftLeaves(TreeNode* root) {
        return root ? dfs(root) : 0;
    }
};

广度优先遍历：

时间复杂度：O(n)

空间复杂度：O(n)

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class Solution {
public:
    bool isLeafNode(TreeNode* node) {
        return !node->left && !node->right;
    }

    int sumOfLeftLeaves(TreeNode* root) {
        if (!root) {
            return 0;
        }

        queue<TreeNode*> q;
        q.push(root);
        int ans = 0;
        while (!q.empty()) {
            TreeNode* node = q.front();
            q.pop();
            if (node->left) {
                if (isLeafNode(node->left)) {
                    ans += node->left->val;
                }
                else {
                    q.push(node->left);
                }
            }
            if (node->right) {
                if (!isLeafNode(node->right)) {
                    q.push(node->right);
                }
            }
        }
        return ans;
    }
};